The present invention generally relates to spread spectrum communication, more particularly to coding and modulation.
Spread spectrum techniques are employed in numerous communication systems, for example, where security, interference rejection and multiple access capability are of particular concerns. The attendant advantages of a spread spectrum system are low probability of interception/detection (LPI/LPD), code-division multiple access, high processing gain to resist jamming, effective elimination of multi-path interference, and countering inter-symbol interference. Spread spectrum technology is deployed in such systems as mobile radio networks and satellite networks.
Spread spectrum is a modulation technique that uses a pseudo-noise (PN) code sequence to xe2x80x9cspreadxe2x80x9d the input signal. As a result, the modulated/encoded signal is transmitted across a bandwidth that exceeds the minimum necessary bandwidth. The receiver locally produces a correlated signal by generating the same PN code sequence, synchronizing its code sequence with that of the received code sequence. The receiver is thus able to track the received encoded signal for proper recovery of the input data stream.
In real communication systems, the channel that the modulated signal propagates over is noisy or otherwise exhibits poor channel response. To improve system performance, error correction codes are used. These error correction codes fall into two broad categories: block codes and trellis codes. Block codes operate on a fixed-length block of source messages in which these fixed-length blocks are independently encoded and decoded. With trellis codes, the input data stream is not partitioned into fixed-length blocks. Instead, the variable length input message is mapped into a code stream of a correspondingly arbitrary length. Trellis codes are more fully discussed later.
In traditional spread spectrum systems, error correction coding and modulation are two separate and distinct sequential processes. FIG. 12 shows a conventional spread spectrum system with error correction coding. The transmitter 1201 includes a channel uncoded source 1203, which generates an input signal. The channel encoder 1205 encodes the input signal from the uncoded channel source 1203 using a conventional error correction technique and supplies the resultant signal to a modulator 1207. The modulator 1207, in turn, outputs a modulated signal to the spreader 1209, which spreads the modulated signal with a PN code before being transmitted over the channel 1211. At the receiver 1213, the received signal is despread by a despreader 1215 with the same PN code. The despread signal then is demodulated by a demodulator 1217. The demodulated signal is finally passed to a Viterbi decoder 1219 for decoding to reconstitute the original uncoded data.
In practical spread spectrum systems, the transmitted signal bandwidth is limited, but the systems are usually required to maintain a minimum amount of processing gain. If the code rate of the channel encoder 1205 is assumed to be xcex1, the bandwidth of the modulated channel encoded data is then increased by 1/xcex1(0 less than xcex1 less than 1). It is noted that the coding gain of the error correction code depends on the code rate. The signal bandwidth, after spreading, is further increased by N, if the PN code length is N. The total bandwidth expansion ratio is thus (1/xcex1)xc3x97N. To improve coding gain, the code rate is decreased, which results in bandwidth expansion. To maintain the same bandwidth, it is required that the length of the PN code be decreased. Consequently, the processing gain of the system is reduced if coding gain is increased. On the other hand, to increase processing gain, the PN code length needs to be increased, making the code rate higher. This will degrade the coding gain. Also, the choice of available code rates is limited. Therefore, in a spread spectrum system with conventional error correction coding, the bandwidth expansion poses a problem for achieving high coding gain and processing gain simultaneously in a bandwidth-limited band in that one is a trade-off for the other. Based on the foregoing, there is a clear need for a mechanism that achieves high coding gain without a decrease in processing gain, or vice versa.
There is also a need for a mechanism that provides flexibility in choice of code rates for acceptable coding gain without bandwidth expansion penalty.
According to one aspect of the present invention, a spread spectrum communication system comprises a transmitter circuit that generates a signal using trellis-coded modulation. The signal is encoded and modulated based upon a plurality of input source symbols. A receiver circuit receives and decodes the signal, thereby, outputting the input source symbols. This system advantageously achieves high coding gain without decreasing processing gain.
Another aspect of the present invention provides a method for communicating in a spread spectrum system. The method includes encoding and modulating a signal based upon input source symbols using trellis-coded modulation. The method further includes transmitting the encoded signal and subsequently receiving the encoded signal. The received encoded signal is then decoded to recover the input source symbols. The above method provides flexibility in choice of coding rates.
In another aspect of the present invention, a spread spectrum transmitter comprises a data source that generates a binary data stream. A trellis-coded modulator is configured for encoding the binary data stream and correspondingly outputting an encoded signal based upon a selected signal constellation. The trellis-coded modulator maps the binary data stream to a signal point in the selected signal constellation by set partitioning. The mapping of the binary data stream is executed using a look-up table. A spreader spreads the encoded signal based upon a pseudo-noise (PN) code sequence. As a result, secure communication can be obtained without sacrificing bandwidth efficiency.
In yet another aspect of the present invention, a spread spectrum receiver comprises a despreader for despreading a received encoded signal that has been encoded and modulated using trellis-coded modulation. A Viterbi decoder decodes the received encoded signal. The receiver also includes memory for storing truncated path metrics associated with the decoding of the received encoded signal. This arrangement provides for a high code gain.
Additional advantages and novel features of the invention will be set forth in part in the description which follows, and in part may become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.